A man believed to be in his 20s is in critical condition after a shark bite at Manly Beach in Sydney's north. He was treated by New South Wales Ambulance officers and taken to Royal North Shore Hospital on Monday evening. NSW Police said members of the public pulled him from the water and gave first aid before help arrived. As a result, all beaches on the Northern Beaches are closed until further notice. The closure will be reviewed regularly, police said. This attack follows two others nearby. On Monday morning, an 11-year-old surfer at Dee Why Beach had his board bitten several times by a shark, which left a 15cm bite mark. The boy was unharmed and helped out of the water. Lifeguards quickly put up shark warnings and began patrolling. The Northern Beaches Council said the beach would stay closed for at least 24 hours. Shark experts say the bite likely came from a bull shark. Authorities noted the shark net was set as usual, but the beach was closed at the time due to dangerous surf. The three recent attacks include a 12-year-old boy who was bitten while swimming in Sydney's east one day earlier. That boy remains in hospital in critical condition after surgery. Police praised his friends, who bravely pulled him from the water and called for help. Supt Joseph McNulty warned against swimming in murky or low-visibility waters, especially after heavy rain flooded the harbour. The fresh water combined with splashing is a "perfect storm" for shark activity. Heavy swells have also prevented the operation of shark warning devices on the coast. The recent attacks are a sharp reminder of the risks, especially after the fatal shark mauling of surfer Mercury Psillakis in September nearby. Authorities continue to monitor the situation and advise caution for swimmers across NSW waterways.